The sonic neuromuscular system of the oyster toadfish (Opsanus tau) is a model of postnatal plasticity and brain and muscle sexual dimorphism underlying sound production and courtship behavior. Neurons in the sonic motor nucleus increase in number for 7-8 years, and sonic muscle fibers proliferate for life. The sonic muscle may be the fastest known vertebrate muscle, commonly contracting in excess of 200 Hz for sound production. Toadfish sounds are ideal for study because their simplicity and stereotypy allow correlation of motor output with neuromuscular development. Since mechanical movement rather than neuronal signalling will limit the sonic system, it is imperative to focus on the sonic muscle as a prelude to a detailed understanding of CNS sonic mechanisms. One major goal of this study will be to examine the basis for sonic muscle fiber proliferation and to describe multiple innervation of sonic fibers, an embryonic trait which persists in adult fish. A second goal will be to relate morphological changes in the developing sonic apparatus to their consequences for sound production. Vibratory patterns and directionality of acoustic radiation from the sonic organ and the consequences of neuromuscular growth and sexual dimorphism on sound production will be measured. Developmental studies will quantify changes in parameters of natural sounds with sounds evoked by stimulating the sonic muscles electrically and the swimbladder mechanically, thus bypassing the brain and sonic muscles. Seasonal changes in the courtship call will be related to androgen levels to test the hypothesis that hormones modulate the output of central pattern generators. A study on intra-male dimorphism in neuron size of the sonic motor nucleus will test the hypothesis that males with large neurons are territorial, produce the courtship boatwhistle call and have high androgen levels compared to males with small neurons who silently obtain "sneak copulations". Functional differences resulting from sexual dimorphisms in the sonic muscle will be examined physiologically by measuring contraction speed and fatigue resistance. In summary this project will relate the mechanics of movement to sound generation in one of the simplest vertebrate systems of sound production and detail proliferative mechanisms underlying its growth and their consequences for behavior.